Integrated circuits or electronic chips are ubiquitous, being contained in many electronic devices used by a person during a typical day, such as in cellular telephones, personal computers, automobiles, and even common household appliances like toasters. A chip includes a semiconductor die, which is made of semiconductor material such as silicon, and in which desired electronic circuitry is formed. For example, a memory chip is a chip containing a die in which electronic circuitry is formed for storing and retrieving data. A chip also includes a package that houses the die and includes pins that provide for electrical interconnection of the chip to external electronic components. Various different types of packages are utilized for chips, with the specific type of package being determined by numerous factors such as required heat dissipation, the physical size of the chip, and the number of interconnections needed from the die to external electronic components. Common packages for chips include single in-line packages (SIPs), dual in-line packages (DIPs), plastic leaded chip carriers (PLCC), Thin Small Outline Packages (TSOPs), pin-grid arrays (PGAs), ball-grid arrays (BGAs), and quad flat packs (QFPs).
In some situations, more than one die is housed in a given package to form what is commonly referred to as a “system in a package” (SIP) device or simply an SIP. The two or more die in this situation must be electrically interconnected, and depending on the type of package this interconnection may present difficulties. These difficulties often occur when using any type of package including a lead frame, such as the DIP, PLCC, TSOP, and QFP packages previously mentioned. For example, a quad-flat pack (QFP) is a package having pins or external leads that project from all four sides of the package. QFP packages are relatively cheap and also are relatively thin (i.e., have a small height) compared to other types of packages, and accordingly may be utilized where cost and height of the package are of concern. A QFP package includes a lead frame and the physical structure of the lead frame and overall QFP package makes the interconnection of multiple dies in such a package problematic.
FIG. 1 is a simplified top view of a portion of a chip including a conventional QFP package containing a lead frame 100. The lead frame 100 includes a die paddle 102 on which two die 104 and 106 are mounted, with the die 104 being a dynamic random access memory (DRAM) and the die 106 being a memory controller in the example of FIG. 1. The die paddle 102 is supported by four support arms 108 (commonly called tie bars) attached to respective corners of the die paddle. Arranged around the periphery of the die paddle 102 are a number of bond fingers 110, several of which are shown along the top, bottom, left, and right edges of the paddle. These bond fingers 110 typically extend from all four sides of the QFP package to form the external leads of the QFP and are also coupled or connected through respective bonding wires 112 to corresponding bond pads 114 on one of the dies 104 and 106. The die paddle 102, bond fingers 110, bonding wires 112, and bond pads 114 are all formed from electrically conductive material, such as a metal, as will be appreciated by those skilled in the art. To simplify FIG. 1, only the bond pads 114 in the upper left-hand corners of the dies 104 and 106 are labeled with the reference indicator 114, although all the small squares contained on each of these dies corresponds to a respective bonding pad. The illustrated bond pads 114 on each of the dies 104 and 106 merely serve to indicate that each die includes such bond pads and the number and arrangement of such bond pads may of course vary for different types of dies.
Each bond finger 110 and corresponding external lead function to route a respective electrical signal to or from the DRAM die 104 and memory controller die 106. Several example signals are shown for individual bond fingers 110 in the example of FIG. 1. For example, some of the bond fingers 110 along the right edge of the die paddle 102 route supply voltage VDD signals to the die 104. Other signals are indicated generically simply as “signal” for some of the bond fingers 110, with the signals on such bond fingers being those required for operation of die 106. In the example of FIG. 1, signals supplied to and from the memory controller die 106 via such bond fingers 110 would include address, data, and control signals.
The die paddle 102 is typically metal and is typically utilized as a ground plane, meaning that the paddle is coupled through bonding wires 112 to bond fingers 110 that receive ground GND signals, as shown for several bond fingers along the right edge of the die paddle. Any bond pads on the dies 104 and 106 that are to be coupled to ground are then simply “down bonded” to the die paddle 102, meaning such bond pads are coupled directly to the die paddle via a corresponding bonding wire 112. Several examples of down bonded ground wires are shown in FIG. 1.
The dies 104 and 106 typically include a number of bond pads 114 that receive the supply voltage signal VDD, as shown for the memory controller die 106 in FIG. 1 along the left, top, and bottom edges of the die. The interconnection of such bond pads 114 and bond fingers 110 through a corresponding bonding wire 112 is simple when the bonding pad is along the edge of the dies 104 and 106 adjacent to the bond finger. For example, routing bonding wires 112 to interconnect bond fingers 110 along the bottom edge of the die paddle 102 to corresponding bond pads 114 along the bottom edge of the memory controller die 106 is straightforward. The same is true for bond fingers 110 along the left and top edges of the die paddle 102 to bond pads 114 along the left and top edges, respectively, of the die 106.
In some situations, however, each of the dies 104 and 106 may include bond pads 114 positioned along the inner edge of the die adjacent to the other die. This is true for both the memory controller die 106 and DRAM die 104 in the example of FIG. 1. For example, the memory controller die 106 includes two bond pads 116 and 118 located along the inner edge of the die. Similarly, the DRAM die 104 includes two bond pads designated 120 and 122 located along the inner edge of this die. Typically, the bond pads 116 and 118 on the memory controller die 106 would be connected to the nearest available bond fingers 110, which are the bond fingers positioned along the left edge of the die paddle 102. Because the bond pads 116 and 118 are positioned along the inner edge of the die 106, relatively long bonding wires 124 and 126 are required to interconnect these bond pads to corresponding bond fingers 110. The same is true for the bond pads 120 and 122, which must be interconnected through respective relatively long bonding wires 128 and 132 to corresponding bond fingers 110 positioned along the right edge of the die paddle 102.
In many instances, the required length of the long bonding wires 124-130 may simply be too long to reliably form such wires. Moreover, even if such long bond wires 124-130 may be formed these wires may undesirably short circuit to other bonding wires 110 during subsequent steps of the manufacture of the QFP package, such as during encapsulation of the structure in a plastic or epoxy resin. Even before such encapsulation, such long bond wires 124-130 also may collapse due to the force of gravity, resulting in undesirable short circuits with other bonding wires 112, or to the die paddle 102 or to the edges of the die 104 and 106. Note there is no similar issue with bond pads 114 along the inner edges of the dies 104 and 106 that are to be coupled to the ground signal GND since these bond pads are simply down-bonded to the die paddle 102.
It should be noted that the bond pads 116-122 cannot be interconnected to bond fingers 110 positioned along the lower or upper edge of the die paddle 102. This is true because in this situation bonding wires 112 running substantially vertically from the vertical stack of inner bonding pads in FIG. 1 may undesirably cross and short circuit to one another or to the other bonding wires running substantially horizontally and interconnecting bond pads and bond fingers.
The structure of a QFP package requires that bonding wires 112 be used to directly interconnect the bond pads 114 and bond fingers 110. This is in contrast to other types of packages such as ball grid arrays where there is an underlying substrate on which the two die 104 and 106 are mounted. This substrate functions like a miniature circuit board and simplifies the routing of the supply voltage signals VDD to required bond pads 114 on the two die 104 and 106.
One approach to solving the problem of providing the supply voltage signal VDD to bond pads 114 along the inner edges of the dies 104 and 106 is to alter the design of dies 104 and 106 so as to reposition the location of the bond pads on each die to be directly across from bond fingers 110. Ideally, however, it is desirable that the same die 104 and 106 could be utilized whether the dies are being placed in a QFP package, a ball grid array package, or any other type of package. Repositioning the bond pads that are presently located along the inner edges of the dies 104 and 106 would make these die unsuitable for use individually in these standard packages. Moreover, this redesign of dies 104 and 106 is relatively expensive and time consuming since it involves the cost of new mask layers used in the die fabrication process and the time it takes to fabricate new die.
Another approach for providing the supply voltage signal VDD to bond pads 114 along the inner edges of the dies 104 and 106 is to relocate the pad locations using a redistribution layer (“RDL”) formed as an additive process on the top of each die. As its name implies, such a redistribution layer redistributes or repositions the locations of underlying bond pads 114 on the dies 104 and 106. With this approach, the bond pads 116 and 118 along the right or inner edge of the memory controller die 106 would be repositioned along the remaining three sides of this die for easy connection to an adjacent bond finger 110 through a relatively short bonding wire 112. The same is true for the bond pads 120 and 122 along the left or inner edge of the DRAM die 104, with these pads being repositioned along the remaining three sides of this die for easy connection to adjacent bond fingers 110. This approach requires the design and actual physical formation of the redistribution layer on the dies 104 and 106. While this method of relocating the bond pads is less expensive and faster than modifying the dies themselves, it is still undesirable. The other three sides may already be fully populated with bond pads and unable to accept new pads. This solution also requires an RDL be used on both dies 104 and 106.
Yet another approach is an interposer layer positioned under dies 104 and 106. The interposer layer functions similar to the substrate previously described for a ball grid array to route a connection for bond pads 116-122 that receive the supply voltage signal VDD to adjacent bond fingers 110 to allow for easy connection to such bond fingers via short bonding wires 112. Once again, this approach is relatively expensive and therefore undesirable, and also increases the vertical height of the QFP package and thereby contravenes one major advantage of a QFP package, namely the small overall height of the QFP package. The same is true for the approach of stacking the two die 104 and 106, which may not be practical if the size of the two die are incompatible and also undesirably affects the heat dissipation and overall height of the QFP package.
There is a need in QFP or other lead frame packages that include more than one die, of interconnecting bond fingers that receive a supply voltage signal to bond pads on the dies that receive the supply voltage signal and which are positioned along inner edges of the die and thus are positioned a relatively great distance from the bond fingers.